This invention relates to color video transmission systems, and more particularly to such systems requiring a local display of the transmitted signal (as, for example, in video conferencing systems).
A color video camera typically generates three signals indicative of the red, green, and blue components of the scanned scene. The digitized version of these signals is referred to as "RGB24." In order for these signals to be kept synchronized during transmission, they are normally encoded into one of several "composite" formats. Generally, these composite formats separate the picture information into a "luminance" signal and into two color difference "chrominance" signals.
For example, the National Television Systems Committee ("NTSC") has a standardized composite format called "YIQ," wherein Y represents the luminance level, and I and Q represent the two color difference chrominance signals. Another similar composite format is the "PAL" format, which originated in Europe, and which produces a different set of chrominance signals "U" and "V" so that this format is often referred to as "YUV" The "YUV" format is normally used in Digital Video Interactive ("DVI") technology due largely to its compatibility with current international standardization efforts in the area of digital video systems.
Once a digitized image has been put into a YUV format, for example YUV24 format, (e.g. with 8 bits/pixel for each of the three components), it is often desirable to employ "color subsampling" to reduce the number of bits of binary data needed to represent the picture signal YUV24 is a known format for video data associated with the PAL (Phase Alternating Line) color television system. A digital YUV24 pixel value (8 bits for each of the Y, U and V components) is directly calculable from a digital RGB24 pixel value (8 bits for each of the red, green and blue components) using the following equations: Y=0.30R+0.59G+0.11B; U=0.62R-0.52G-0.10B; V=-0.15R -0.29G+0.44B. For instance "YUV 9 subsampling" involves assigning one pixel's color information to 15 adjacent pixels so that 16 pixels' color information is contained in only 16 bits of binary data. By simultaneously keeping the luminance signal data in the 8 bit per pixel format the total number of bits/pixel for this subsampled arrangement is only 9 instead of the original value of 24. This approach still yields an acceptable color picture, because the human eye has relatively poor acuity for detecting color changes in an image (as opposed to luminance changes). So for many practical applications the YUV9 format is quite useful.
It should be noted that the YUV9 format allows for inclusion all of the original RGB24 colors (i.e. 2.sup.24 =16,777,216 of them), which are also the possible YUV24 colors, but just in a different format. So the YUV9 format has all these colors, but is limited to displaying them in 16 pixel groupings, which results in four times less distinct color transitions in both the x and y directions of a video display. Once again, in most uses this is not a major problem due to the limitations of the human eye.
Processing these large variety of colors requires a large amount of computer capacity and computer time. It would therefore be desirable, in certain applications such as local display of preview images, to employ a simplified technique to produce color images with a much smaller number of basic colors included that would nonetheless serve the desired purpose quite well.
It is therefore an object of the instant invention to provide a novel simplified technique for creating an acceptable local preview image in an image transmission system.
It is a further object of the instant invention to produce an acceptable local preview image without the need for local decompression of that image at the transmitting station.
It is yet a further object of the invention to accomplish the production of an acceptable local preview image with little local processing needed other than simple memory moves in a look-up table.